Ultra-high molecular weight (UHMW) polyolefin, especially UHMW polyethylene, with a molecular weight of 1×106 g/mol (gram per mole) or higher, is a commercially desired polymer because of its toughness and high impact strength, which is mainly influenced by the length of the molecular chain. The long molecular chain serves to transfer load more effectively to the polymer back-bone by strengthening the intermolecular interactions. Further, such polymers with long molecular chains are highly resistant to corrosive chemicals with the exception of oxidizing acids, and have extremely low moisture absorption capability, have a very low coefficient of friction, are self-lubricating and are highly resistant to abrasion. Based on the aforementioned unique properties, UHMW polyolefins have a variety of important commercial applications such as in protection fabrics, joint replacement materials and micro porous films.
Various conventional olefin polymerization processes are generally carried out in the presence of either homogenous or heterogeneous Ziegler-Natta catalyst systems. A heterogeneous Ziegler-Natta catalyst system is prepared by the activation of a Magnesium-Titanium (Mg—Ti) base by adding an organo-aluminum co-catalyst. To improve the activity of the catalyst and also to achieve and enhance certain polymer characteristics, either an internal electron donor compound is added to the Mg support before the activation of the catalyst or an external electron donor compound is added after the activation of the catalyst. Both the donors are mainly added to stabilize the geometry of the titanium (Ti+3) ion which governs the molecular weight of the polymerized olefin, besides controlling certain other polymer characteristics. In comparison, a homogeneous single site catalyst system includes a complex of different organic ligands with elements such as Titanium (Ti), Zirconium (Zr), Hafnium (Hf) and the like which can result in metallocenes where cyclopentadiene is used and non-metallocenes where cyclopentadiene is absent. The homogeneous single site catalyst system further includes a co-catalyst such as methylaluminoxane (also known as methylalumoxane). However, preparation of such homogeneous single site catalyst systems is cost-ineffective because of the use of co-catalysts that are generally expensive. Further, the use of methylaluminoxane as the co-catalyst results in-fouling of the polymerization unit, in most of the cases, as large quantities are needed to activate the catalyst systems. Consequently, use of the homogeneous single site catalyst system is expensive to maintain the polymerization unit.
As Titanium (Ti+3) ions play an important role in the polymerization process, efforts have been directed towards modification of Ziegler-Natta catalyst systems, especially by using electron donors which play an important role in the stabilization of Ti+3 ions in an activated catalyst and facilitate an increase in the length of the polymer chain. A variety of compounds have been explored as internal and/or external electron donors for modification of the heterogeneous Ziegler-Natta catalyst system.
U.S. Pat. No. 4,962,167, US2011/0159287, and U.S. Pat. No. 6,559,249 disclose polymerization processes catalysed by Ziegler-Natta catalyst compositions including a Mg-ethoxide-Ti-tetrabutoxide base with an organo-aluminum co-catalyst, with electron donor groups such as alcohols, ethers, esters, silanes and amines, for the preparation of UHMW polyethylene. The disclosed processes depict an increase in bulk density, intrinsic viscosity as well as molecular weight of the resulting UHMW polyethylene with a narrow molecular weight distribution.
Further, U.S. Pat. No. 7,371,806 discloses a process for the preparation of ethylene co-polymer with olefins, having molecular weight distribution higher than three using a Ziegler-Natta catalyst system that includes Magnesium Chloride (MgCl2) and Titanium tetra-alkoxide (Ti(OR)4) base with specific 1,3-diethers and an organo-aluminum co-catalyst. The process also demonstrates that addition of a small amount of an electron donor reduces the concentration of the polymer chains having molecular weight less than 1×106 g/mol in the resulting product.
WO2012119953 discloses processes for the preparation of UHMW polyethylene by blending two low molecular weight polyethylene resins, prepared by polymerization of ethylene in presence of a Ziegler-Natta catalyst composition including Ti supported on Mg-alkoxide along with triethyl aluminum (TEAL) as a co-catalyst. The catalyst is prepared by the addition of halogenating/titaning agent in three successive addition steps followed by addition of co-catalyst.
CA2057688 discloses a process for the preparation of polyolefins, such as polyethylene, using a Ziegler-Natta catalytic composition that includes a pro-catalyst based on a titanium compound (preferably TiCl4 on MgCl2 carrier) and triethyl aluminum as a co-catalyst and an organo-silane compound (preferably 4-t-butyl-phenyl-trimethoxysilane, biphenyl-trimethoxysilane or methyl(phenoxyphenyl)-dimethoxysilane) as an external donor. The catalyst is prepared by mixing triethyl aluminum and 4-t-butyl trimethoxysilane as an external donor into dried heptane followed by addition of the solid pro-catalyst. The catalyst composition thus, obtained is directly added to the polymerization reaction.
EP607771 discloses a process for the preparation of polypropylene by polymerization of propylene using a Ziegler-Natta catalyst composition that includes Mg—Ti base, triethylaluminum as a co-catalyst and di-t-butyl dimethoxysilane as an external donor. The Ziegler-Natta catalytic composition is prepared by addition of a mixture comprising donor and the co-catalyst to the Mg—Ti base that is then added to the polymerization process.
EP1877450 discloses a process for the preparation of an olefinic polymer comprising contacting at least one olefinic monomer with a carbon chain consisting of more than three carbon atoms (C3+) in the presence of Ziegler-Natta catalyst composition consisting of Titanium supported on Mg support, trialkyl aluminum as a co-catalyst and dicyclopentyl dimethoxysilane as a first electron donor and a second electron donor selected from the group consisting of methyl trimethoxysilane, methyl triethoxysilane, dimethyl dimethoxysilane and combinations thereof. The catalytic composition is prepared by the addition of the co-catalyst followed by the addition of the mixture comprising donors, to the Mg—Ti base. The resulting catalyst composition is then added to the polymerization process.
WO1997043321 discloses a process for the preparation of polyolefin, more specifically polypropylene, in presence of Ziegler-Natta catalyst composition comprising Ti supported on MgCl2 base, (TEAL) as a co-catalyst and two or more external electron donors that consist of a mixture of at least two silane compounds. The catalyst composition is prepared by the addition of a co-catalyst followed by the addition of a mixture comprising external donors to the Mg—Ti catalyst support. The resulting catalytic composition is then added to the polymerization process.
KR1997700702 discloses a process for the preparation of polyolefin in the presence of a Ziegler-Natta catalyst composition where Ti supported on MgCl2 base, (TEAL) as a co-catalyst and two external electron donors consisting of a mixture of at least two silane compounds out of which one is dominant. The catalyst composition is prepared by the addition of a mixture of a co-catalyst and external electron donors to the commercially available Ti—Mg catalyst followed by the addition of the catalytic composition to the polymerization reaction.
BRPI0209871, IN231212, MX275899, IN239275, IN20030196P61, IN200301968, MX2003010690, HU200400057, AU2002316912 and AU2002302619A1 disclose processes for the preparation of polypropylene film used for coating purposes by employing Ziegler-Natta catalyst composition that consists of titanium-containing solids components, an organo-aluminum, magnesium or titanium compound as a co-catalyst and an external donor of formula RxR′ySi(MeO)4-x-y′.
JP2010537028 discloses a process for the preparation of a co-polymer of ethylene in the presence of a Ziegler-Natta catalyst composition comprising titanium, magnesium or halogen, an aluminum alkyl compound as a co-catalyst and silane compounds such as methyldimethoxysilane, methyldiethoxysilane or trimethoxysilane as external electron donors. After the addition of the trialkyl aluminum co-catalyst, the activated catalyst is subjected to pre-polymerization followed by the addition of the silane compound. The resultant catalytic mixture is then added to the polymerization process.
Furthermore, Basell in U.S. Pat. No. 7,196,152 and U.S. Pat. No. 7,371,806 has also attempted the use of donors in a process for the preparation of high density polyethylene with the objective of reducing the wax content in the polymer.
From the foregoing, it is observed that a large number of polymerization processes with heterogeneous Ziegler-Natta catalyst systems have been developed in recent years. However, none of the aforementioned processes demonstrate any significant increase in the molecular weight and other physicochemical properties of the polyolefin products. Thus, there exists a long felt need for specifically engineering heterogeneous Ziegler-Natta catalyst systems in order to increase the molecular weight of the resultant polyolefin significantly.